As a method for diagnosing breast cancer, liver cirrhosis, vascular disorder, and the like, there is a method for diagnosing the hardness inside a test object based on ultrasound echo signals (elastography technique), the method substituting for palpation by a doctor. In the diagnosis of the hardness according to the elastography technique, an engaged person presses a probe against a surface of the test object and exerts compression thereon, thereby causing displacement in tissue inside the living body (hereinafter, this technique is referred to as a conventional method). According to echo signals from the tissue of the living body, before and after compressed by pressure, it is possible to estimate the displacement in the compressed direction and obtain strains corresponding to spatial differential information of the displacement. Furthermore, based on the strain and stress, a value relating to the hardness is calculated, for instance, Young's modulus. In the aforementioned conventional method, there is a problem that a target for imaging is limited to an organ that is located easily compressed from the body surface. By way of example, there exists a slip surface as an intervening layer between the body surface and the liver, and it is difficult to exert compression sufficient enough to cause displacement in the liver. Therefore, according to the conventional method, it is difficult to calculate the hardness inside the liver by the use of the elastography technique.
In view of the situation above, there is a technique that the inside of the test object is exposed to a focused beam of ultrasound waves, thereby applying radiation pressure to the inside of the test object, allowing a target tissue to be displaced while diminishing the influence of the intervening layer, and the hardness thereof is diagnosed. By way of example, such technique as described above includes the ARFI (Acoustic Radiation Force Imaging) described in the Patent Document 1 and the SSI (Supersonic Shear Imaging) described in the Non Patent Document 1. Those techniques estimate an amount of tissue displacement which occurs in a proceeding direction of the focused beam, so as to calculate the Young's modulus, or estimate a velocity of a shear wave which is produced according to the displacement, in the direction vertical to the proceeding direction of the focused beam, thereby calculating a shear elastic modulus. Since the shear elastic modulus covers a wider range of values possibly obtained depending on various types of tissue, relative to the Young's modulus, it is possible to expect more precise diagnosis. By using those techniques described above, there is an effect of reducing the influence of the intervening layer such as the slip surface described above, and additionally, since ultrasound waves generate displacement in the tissue, diagnosis with less dependence on manual procedure is expected.
On the other hand, the Non Patent Document 2 describes the possibility that the test object is exposed to two focused beams having different frequencies, receiving a beat signal being a difference of the frequencies, and the hardness of blood vessels can be diagnosed based on a peak value of a spectrum of the beat signal. The technique here assumes the shape of the blood vessel as a ring, and adopts that the natural vibration frequency of the ring depends on the Young's modulus.